support loading and saving models

[guanyilun]

support saving models with syntax of

@emulate(samples={
    'a': np.random.rand(100),
    'b': np.random.rand(100)
}, backend='sklearn', save="test.pkl")
def test_em(a=1, b=1):
    x = np.linspace(0, 10, 100)
    return np.sin(a*x) + np.sin(b*x)

and then loading it with

@emulate(backend='sklearn', load="test.pkl")
def test_em(a=1, b=1):
    x = np.linspace(0, 10, 100)
    return np.sin(a*x) + np.sin(b*x)

[codecov-commenter]

Codecov Report

Base: 0.00% // Head: 0.00% // No change to project coverage :thumbsup:

Coverage data is based on head (66930c4) compared to base (a4a6508). Patch coverage: 0.00% of modified lines in pull request are covered.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #4 +/- ## ====================================== Coverage 0.00% 0.00% ====================================== Files 3 3 Lines 172 223 +51 ====================================== - Misses 172 223 +51 ``` | [Impacted Files](https://codecov.io/gh/guanyilun/galilei/pull/4?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yilun+Guan) | Coverage Δ | | |---|---|---| | [galilei/emulator.py](https://codecov.io/gh/guanyilun/galilei/pull/4?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yilun+Guan#diff-Z2FsaWxlaS9lbXVsYXRvci5weQ==) | `0.00% <0.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yilun+Guan). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yilun+Guan)

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